Ephemeroptera are actually "upwinged flies" of which the mayfly is the best known and one of the largest. There are about 50 species in the UK, 1500 worldwide. They have two or three tails and two large upright wings. The largest in the UK is the danica at 3/4 inch long (2cm), a 5/8 inch wingspan and inch-long tails, it is also one of the most common. Their brief adult life leads to their naming, "ephemeral".
After mating the female will lay her eggs on the water either by dipping the tip of her abdomen in the water or by settling on the water for a time. Several visits may be made until all the eggs are laid after which the female will lie spent on the water.
After the eggs hatch, the nymph stage lasts from one to two years. The nymphs feed on algae and underwater vegetation and from late Spring until the Autumn fully developed nymphs will swim to the surface to progress to the next stage.
At the surface, the nymph case splits open and the sub-adult (or sub-imago) fly emerges. This will stay on the surface of the water while the wings dry and within a day the dun becomes a spinner.
This is the adult stage, the spinners form swarms above the water and mate. After mating, the males die, the females lay their fertilized eggs and then die, the dead adults lying on the water as spent spinners. From emergence of the dun to spent spinner only takes few days, in warmer weather the whole thing can be over within a single day.